This invention relates to a load cell for measuring tensile load and changes in tensile load in a tendon. The invention has particular application to the monitoring of loads and changes in loads in tendons which extend through concrete structures and are tensioned to produce an initial compressive stress in the concrete structure. Such tendons may be constructed of a number of strands or "wires" which extend through a casing in the concrete structure and have their ends connected to stop members positioned on either side of the portion of the concrete structure being acted upon by the tendon. A tensile stress is applied to the tendon resulting in a corresponding compressive force on the concrete structure. The concrete structure is thereby post-tensioned in compression by the amount of tension that is present in the tendon which in turn will depend upon the average of the tensile loading of each of the wires comprising the tendon.
From the foregoing, it is apparent that the tendon acting through the stop members applied a compressive force to the concrete structure, and when a tensile loading is applied to the concrete this compressive force must be overcome before tensile stress develops in the concrete. Thus any changes in the tensioning of the tendon will alter the strength of the entire structure in tension. In many structures, such as secondary containment vessels for nuclear power plants, it is highly desirable to at least periodically monitor the tensioning in the tendon to make sure that no changes have occurred which would indicate a lessening of overall tensile strength of the reinforced structure.
Heretofore it has been common practice to periodically monitor the tensioning of the tendon by a post tensioning procedure involving the application of a tensile load to the stop member to determine the force required to overcome the tension in the tendon and to move the stop member away from the compressed structure. The force required to accomplish this would then be just slightly greater than the tension in the tendon being monitored. Such a procedure is cumbersome and time consuming, and it could be dangerous if there is a weakened wire in the tendon. Other procedures have been attempted. One such alternative procedure is to apply a number of bonded strain gages to the exterior wall of a spacing structure which has been positioned between one of the stop members and the concrete structure and has been placed in compression by the initial tension in the tendon. The readings of the strain gages would be averaged. Another alternative procedure involves the use of so-called "vibrating wires". The frequency of vibration of each wire varies with the tension in the wire. Such wires may be placed at various positions and again the readings averaged. However, all of these alternative procedures are complicated, cumbersome, and require an averaging of the readings taken.
The post-tensioning load cell constructed in accordance with this invention provides a less complicated, less expensive means of measuring the tensile load and variations in the tensile load on the tendon. The load cell is easy to install and easy to read and automatically averages the loads and changes of loads in the individual wires of the tendon. An important feature of the invention is that measurements are not affected by eccentricity in the components of the tendon or the components of the compressed structure or the components of the load cell.